1. Field of the Invention
The present invention is directed to a method for making a prepreg of an epoxy resin impregnated fiberglass substrate, from which an electrical laminate is formed.
2. Description of the Prior Art
Various combinations of materials are utilized to make an electrical laminate for a printed circuit board. For example, as it has already been known, a method for making the electrical laminate comprises impregnating a substrate such as fiberglass cloth with a resin such as epoxy resin, drying to partially cure the resin impregnated substrate into a prepreg, forming sheets of the prepreg into a stacked array, laminating a metal foil on at least one side of the stacked array if necessary, and then heat-curing the stacked array with the metal foil under pressure in order to form the electrical laminate. However, heat resistance, drilling workability and humidity resistance of the thus obtained electrical laminate is not enough to form a small-sized or more dense integrated circuit. It is also required that adhesive strength between the prepregs or between the metal foil and the prepreg is improved. For improving the above problems, U.S. Pat. No. 4,868,059 describes that poly-functional epoxy resin and dicyandiamide as a curing agent are used to form a prepreg. The prior electrical laminate demonstrates high glass transition (Tg) temperature, strong adhesive strength between a copper foil and the prepreg, and also excellent heat resistance for soldering. However, the prior laminate also shows poor electrolytic corrosion resistance and humidity resistance. This undesirable performance results from dicyandiamide being used as the curing agent. On the other hand, another prior electrical laminate is made by using diaminodiphenylsulfone (DDS), diaminodiphenylmethane (DDM), or an alkylated or monohalogenated DDM, etc., as the curing agent. The prior electrical laminate has high Tg, but it also shows poor humidity resistance. A prepreg made in accordance with this prior art has short shelf life. An electrical laminate of another prior art is made through using an imido resin modified epoxy resin. The prior electrical laminate demonstrates excellent heat resistance, but there are problems with respect to low adhesive strength between prepregs and between a copper foil and the prepreg, and also expensive production cost for preparing the laminate. U.S. Pat. No. 4,868,059 also describes that a phenolic resin having a phenolic hydroxy group, for example, such as phenol novolac resin and cresol novolac resin, is utilized as the curing agent. The prior laminate has excellent humidity resistance and, heat resistance for soldering. A prepreg made in accordance with this prior art has long shelf life. However, there are problems with respect to low Tg, discoloration of the laminate which is caused by heating it or exposing it in ultraviolet light. The laminate also shows poor adhesive strength between the copper foil and the prepreg. Therefore, it is difficult to improve all problems described above, that is, a method for making a prepreg for forming an electrical laminate which has well-balanced properties such as excellent heat resistance, humidity resistance and electrolytic corrosion resistance, strong adhesive strength between the prepregs and between the metal foil and the prepreg, etc., has not been known yet.